Condon Report: 190-N. Detroit, Mich., March 1953, about 1000 to 1100 LST (exact date and time unknown). Weather: "perfectly clear." A USAF pilot and a radar operator, flying in an F-94B fighter on a practice training mission, were directed by GCI radar at Selfridge AFB to intercept some unknown targets which appeared to be over downtown Detroit. The pilot and radar operator looked in that direction and saw "tiny specks in the sky, which appeared to look like a ragged formation of aircraft."

The aircraft at this time was about 30 mi. NW of downtown Detroit, and the targets "appeared to be over the city's central section." The pilot turned the aircraft to an intercept course. During this time, perhaps "three or four minutes," the objects were visible to the pilot as "a ragged formation traveling slowly in a westward direction;" the objects appeared to be "a little lower than our aircraft." The pilot started his intercept run under full military power, without afterburner, at approximately 500 mph.

The pilot recalls thinking several times that details of the unknowns, like wings, tails, etc. should have "popped out" as they approached, so that identification could be made, but they did not. The ground radar had both the F-94B and the unknowns "painted as good, strong targets." The unknowns could still not be identified, but "seemed to get a little larger all the time."

The F-94B's radar operator began to get returns and "thought he was picking up the targets." The pilot looked at his instruments to see if he could "inch out a little more speed without going into after-burner," and when he looked up again "every last one" of the objects was gone. The pilot asked GCI where the UFOs were, and was told they were still there, "loud and clear." They continued to fly headings given by GCI right into the center of the targets, flying and turning in "every direction," but there was nothing in sight. The pilot states: "Gradually the targets disappeared from ground radar after we had been amongst them for three or four minutes." The F-94B then returned to base.

Since the exact date of this sighting is unknown, no applicable meteorological data are available. Any explanation of this incident must therefore remain speculative in nature. If the UFOs are considered to have been material objects, then they would have had to have shifted position some tens of miles in the "two to four" seconds while the pilot was looking down at his instruments. This does not explain why they continued to appear on the ground radar. The only admissable hypothesis would seem to be that they became invisible as the fighter approached, but this does not account for the fact that they could not be picked up on airborne radar while the aircraft was searching the area.

There is one hypothesis that seems to fit all of the observed facts: that the "ragged formation" was actually an inferior mirage (see Section VI, Chapter 4). The angular conditions are satisfied: the objects appeared "slightly below the level of the aircraft," and reflections of the sky above the horizon would seem dark when seen projected against the hazy sky directly over the city. A layer of heated air, trapped temporarily below a cooler layer by a stable vertical wind shear, could produce a wavy interface that would reflect the sky in a few spots. This phenomenon is quite similar to the familiar road mirage. Like, a road mirage it suddenly disappears when one gets too close and the viewing angle becomes either too large or too small.

If the warm air below, the source of which would presumably have been the downtown area of Detroit, were also considerably moister than the cooler air above as is quite probable, then the radio refractive index would decrease quite suddenly across the inter-face. This would tend to produce anomalous propagation effects, including false echoes, on radar, and would explain why ground radar could continue tracking the unknowns when the pilot and airborne radar operator could no longer see them. The airborne radar, being immersed in the layer would probably not receive AP echoes of any duration other than, perhaps, occasional random blips. After the aircraft had thoroughly mixed the opposing air currents by flying repeatedly through the interface as it searched for the targets, the ground radar returns would gradually fade away. This corresponds to what was actually observed.

In summary, without the data to make a more definitive evaluation of this case, the most likely cause seems to be a combined radio-optical mirage as described above. If so, this is another example of a natural phenomenon so rare that it is seldom observed: for a 0.25° critical mirage angle, the temperature contrast required is on the order of 10° or 15°C in the space of about 1 cm.

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